Sand dune migration as a factor of geoheritage loss: Evidence from the Siwa Oasis (Egypt) and implications for geoheritage management

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Proceedings of the Geologists’ Association xxx (2019) xxx–xxx

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Sand dune migration as a factor of geoheritage loss: Evidence from the Siwa Oasis (Egypt) and implications for geoheritage management Kholoud M. AbdelMaksouda , Wael M. Al-Metwalyb , Dmitry A. Rubanc,* , Natalia N. Yashalovad a

Natural Resources Department, Institute of African Research and Studies, Cairo University, Giza, Cairo, 12613, Egypt Department of Geography, Institute of African Research and Studies, Cairo University, Giza, Cairo, 12613, Egypt K.G. Razumovsky Moscow State University of Technologies and Management (the First Cossack University), Zemlyanoy Val 73, Moscow 109004, Russia d Cherepovets State University, Sovetskiy Avenue 10, Cherepovets, Vologda Region, 162600, Russia b c

A R T I C L E I N F O

A B S T R A C T

Article history: Received 29 March 2019 Received in revised form 1 July 2019 Accepted 4 July 2019 Available online xxx

Anthropogenic damage of geoheritage is documented widely, but natural processes can also lead to geoheritage loss. For instance, sand dune migration causes submergence of unique geological and palaeontological sites in desert environments of the Sahara. The Siwa Oasis in the Western Desert of Egypt boasts rich geoheritage, which is represented in many localities. Three of them in the southern part of the oasis are outcrops of highly-fossiliferous limestones. Palaeontological, sedimentary, palaeogeographical, and geomorphological types of geoheritage are recognized there. Sand dune activity on the study area is registered both visually and with remote sensing techniques. Denudation and destruction of naturally-exposed rocks is documented. Evidence of outcrop submergence with sand is found in all cases. The localities are situated in the pathway of rapid (up to ~10 m/yr) dune migration. One locality may disappear within one–two years. Sand dune migration has to be considered as a factor of geoheritage loss in the Siwa Oasis, and the relevant protection of the studied localities is necessary. Geopark creation and improvement of water use in the oasis can also help significantly, as well as the reference to archaeological experience of excavation and protection of heritage sites submerged by sands. More generally, geoheritage conservation should be integrated with a program for sustainable oasis development. © 2019 The Geologists' Association. Published by Elsevier Ltd. All rights reserved.

Keywords: Desert environment Geological resources Palaeontological sites Remote sensing Western Desert

1. Introduction Geological heritage (geoheritage) is the entity of peculiar and chiefly unique geological features valuable to society, and it is important resource for sustainable development (Prosser et al., 2006; Gray, 2013; Prosser, 2013; Brilha, 2016; Henriques and Brilha, 2017; Reynard and Brilha, 2018; Gordon et al., 2018). Geoheritage recognition and conservation are an integral part of nature conservation, as well as provide income from new kinds of tourism activity. Therefore, protection and rational use of this resource are necessary. Geoheritage loss because of anthropogenic pressure has been documented in many parts of the world,

* Corresponding author. E-mail addresses: [email protected], [email protected] (K.M. AbdelMaksoud), [email protected], [email protected] (W.M. Al-Metwaly), [email protected] (D.A. Ruban), [email protected] (N.N. Yashalova).

including Northeast Africa (El-Asmar et al., 2012; AbdelMaksoud et al., 2018) and the Middle East (Şengör and Lom, 2017). Some approaches for the analysis of this loss have been proposed earlier (Ruban, 2010a). However, the negative influence of natural processes on geoheritage may be similarly or even more significant, although this issue has been studied only occasionally (e.g., Mansur et al., 2013; Ferrero and Magagna, 2015; Park and Park, 2017). The examples of geoheritage damage by climate changes, sea-level rise, river and coastal erosion, etc, can be found in the works by Prosser et al. (2010); Sharples (2011); Brown et al., 2012; Brazier et al. (2012), and Wignall et al. (2018). These examples demonstrate the diversity of the negative influence of natural processes on geoheritage, although this diversity is yet to be fully realized. Desert environments are often ideal for displaying geoheritage that is not masked by vegetation. But these environments can also be very destructive to geoheritage. Denudation of well-exposed landforms results in destruction of unique features, and rock debris and sand may cover these features to make them fully invisible.

https://doi.org/10.1016/j.pgeola.2019.07.001 0016-7878/© 2019 The Geologists' Association. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: K.M. AbdelMaksoud, et al., Sand dune migration as a factor of geoheritage loss: Evidence from the Siwa Oasis (Egypt) and implications for geoheritage management, Proc. Geol. Assoc. (2019), https://doi.org/10.1016/j.pgeola.2019.07.001

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The relevant geoheritage loss may be significant on regional and national scales. World-class geoheritage is found in many domains of the Sahara Desert. Significant attention has been paid recently to the importance of this resource to tourism growth and the relevant socio-economical development in Egypt (Ólafsdóttir and Tverijonaite, 2018). Particularly, the Siwa Oasis in the Western Desert possesses significant and diverse geoheritage that has been characterized comprehensively by Sallam et al. (2018). This geoheritage of the entire oasis includes stratigraphical, palaeontological, hydrological, and some other notable features. Particularly, desert sand dunes occupying large territories to the south of this oasis are considered among unique sedimentary features (Sallam et al., 2018). However, migration of these sand dunes may trigger significant geoheritage loss. A new field investigation in the Siwa Oasis coupled with application of remote sensing techniques examines this negative factor. 2. Geological setting The Siwa Oasis corresponds to the similarly-named depression that is located in the western part of Egypt (Fig. 1). This was characterized generally by Stanley (1912) and recently by Embabi (2004); Tawfik and Tolba (2014); Tawfik (2016), and Sallam et al. (2018). Geographically, it is situated in the Western Desert and belongs to the geographical region of the Sahara. The depression is elongated and oriented west–east. Its length is ~100 km, and its width reaches 50 km; the area is 3400 km2. The borders of the

depression reach elevations of 150–200 m, whereas its central part lies below sea level (Fig. 1). The climate is continental arid. Several lakes and springs exist in the depression and sustain the oasis (Döring and Nuding, 2002). Sand dunes dominate the territory to the south the oasis. These dunes represent the margin of the socalled Great Sand Sea (Haynes, 1982; Besler, 2000,2008). Desertification in the oasis is active (Ghonaim and Gabriel, 1980; Misak and Draz, 1997). Geologically, the Siwa Oasis is a sedimentary basin controlled by regional tectonic structures (Issawi et al., 2009). The Moghra and Marmarica formations (siliciclastic-dominated and carbonatedominated, respectively) of Miocene age crop out widely, as well as the Quaternary sediments (see overview in Sallam et al., 2018). Stratigraphical sections, fossil sites, sand dunes, sabkha deposits, saline lakes, uncommon landforms, etc. are recognized as geoheritage features (both static and dynamic) ranked from local to global (Sallam et al., 2018). Of particular interest are shell beds of the Marmarica Formation that have been studied by El-Sabbagh and El Hedeny (2016) in the northern part of the oasis. These provide important information about fossil bivalves, bivalvedominated palaeoecosystems, fossiliferous sedimentary rocks, and mid-Miocene depositional environments. These are relevant to palaeontological, sedimentary, and palaeogeographical types of geoheritage (sensu Ruban, 2010a). The entire oasis and its surroundings should be considered as national geodiversity hotspot (in terms of Ruban, 2010a; Bétard et al., 2018; Bétard and Peulvast, 2019). The latter is a significant resource for

Fig. 1. Location and digital elevation model of the study territory (see text for explanation and data source). Black circle marks the study area; see Table 1 for locality positioning.

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Table 1 Geological localities of the Siwa Oasis considered in this study. Unique features (geoheritage types – see Ruban, 2010a)

Locality

GPS coordinates x

y

A B C

25.469444 25.459778 25.583028

29.113611 29.068111 29.011222

exploitation for the purposes of tourism (Sallam et al., 2018), but its uniqueness also means that geoconservation procedures, including protection from anthropogenic and natural damages, are required. 3. Methodology 3.1. Field investigations Three localities (labeled A–C) situated to the south of the Siwa Oasis were examined (Table 1). These represent the Marmarica Formation, including the shell beds of its Oasis Member. Although the geological map by Conoco Coral and the Egyptian General Petroleum Corporation (1987) reproduced later by Salvini et al. (2015) restricts the distribution of the Marmarica Formation to the northern part of the oasis, our new studies provide evidences of the existence of its outcrops in the southern part where these outcrops are partly covered by sand dunes. Undoubtedly, these localities are part of the geoheritage potential of the Marmarica Formation. These are comparable to the other localities described by ElSabbagh and El Hedeny (2016) in the northern part of the oasis. Field investigations for the present study included general examination of the localities and evaluation of their potential as geoheritage sites. Approaches and criteria proposed earlier by Ruban (2010a) and implemented regionally by Sallam et al. (2018) were taken into account. For instance, the local, regional, national, and global uniqueness of geological features is distinguished on the basis of their spatial rarity (e.g., rarity on the scale of the oasis is a local uniqueness). Special attention was paid to spatial relationships of the localities to sand dunes because these localities are situated at the border of the Great Sand Sea.

Palaeontological, sedimentary, palaeogeographical Palaeontological, palaeogeographical, geomorphological Palaeogeographical

enhancement tool to correct the digital elevation model (DEM) error like no data (NULL) and sink pixels; the contour file was then extracted to detect the study area. Specifications of the multi-spectral satellite imagery data acquired for this study are given in Table 3. These data were pre-processed in Arc-GIS 10.3 software for layer stacking, spectral enhancement, radiometric enhancement, and spatial enhancement. The enhancement functions include layer stack function, which is used to combine the bands of the image and to create multispectral image in order to apply different colour composites in both true colour and false colours for visual interpretation. Then radiometric and spectral enhancement techniques were applied using the more common techniques (contrast stretch, density slicing, edge enhancement, and spatial filtering) to improve the quality of coloured image and to avoid inadequate information for interpretation. Finally, resolution merge as a spatial enhancement method was applied to the image; this function enables integration of imagery of different spatial resolution. Higher resolution

3.2. Remote sensing Remote sensing techniques are very diverse (Lillesand et al., 2015; Prost, 2014), and some of them are powerful in geoheritage identification, evaluation, and monitoring (El-Asmar et al., 2012; Hjort and Luoto, 2012; Corumluoglu et al., 2015; AbdelMaksoud et al., 2018; Wang, 2018). Some techniques have been used in the Saharan dune investigations (Boulghobra, 2016; Boulghobra and Dridi, 2016; Zaoui et al., 2017). For the purposes of this study, three approaches were used (Fig. 2): 1) construction of regional digital elevation model (Fig. 1); 2) satellite images processing to delineate sand dunes; 3) measurement of temporal changes in dune location in order to establish direction and speed of dune migration. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Global Digital Elevation Model (GDEM) were acquired as 4 scenes/tiles (N28 E25, N28 E26, N29 E25, N29 E26). Specifications of the ASTER GDEM imagery data acquired for this study are given in Table 2. These data were pre-processed in Arc-GIS 10.3 software for mosaic to one scene and re-projected to match the geographic coordinate system (UTM coordinate system – Datum: WGS 1984, Zone 36 North) for Integrity with the Landsat 8 OLI Scene. The fill enhancement function was applied as

Fig. 2. Interpretation of remote sensing data used in this study.

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Table 2 Specifications of the ASTER GDEM imagery data acquired for this study. Parameter

Value/meaning

Tile size Posting interval Geographic coordinates system DEM output format

3601*3601 (1 by 1 ) 1 arc-second Geographic latitude and longitude

Source



in the study area to the established direction and speed of dune migration. 4. Results



4.1. Geoheritage features and visual evidence of their natural damage

Geo-TIFF, signed 16 bits, 1 m/DN referenced to WGS 1984/EGM 96 geoids https://www.gdem.aster.ersdac.or.jp/

The studied localities are small-sized areas with natural outcrops of the Marmarica Formation (Figs. 3–5). These represent highly-fossiliferous limestones deposited in shallow-water environments where rich palaeoecosystems flourished. This formation taken entirely has a significant geoheritage potential being nationally unique in regard to its sedimentological peculiarities, fossil content, and palaeoenvironmental interpretations. Generally, it represents a carbonate factory of the wide shelf of the Miocene Mediterranean Sea where bivalve community growth in tropical conditions resulted in shell debris deposition in a shallow sea. Studying this formation informs about the Miocene evolution of the Mediterranean periphery of the African Plate. As such, the localities of the Marmarica Formation reflects its uniqueness and, in the other words, provides some particular unique features. These include very abundant bivalve fossils (Fig. 3B), shell beds (Fig. 4C), and specific facies. One should also note abundant ferricrete-linked iron concretions (Fig. 3B), trace fossils, and unusual landforms such as small natural arches (Fig. 4D). Such localities are rather common in both Egypt (Sallam and Ruban, 2017) and the rest of the world (e.g., Gómez-Alday and Elorza, 2003; Ruban, 2010b; Dominici and Zuschin, 2016). However, shell beds are of utmost importance for palaeontological, palaeoecological, and taphonomical studies because of fossil

imagery is generally single band (panchromatic band 8 as 15 m data), and multispectral imagery generally has lower resolution (30 m data) to produce best enhanced merged imagery (highresolution, multispectral imagery) to improve the interpretability of the data by having high resolution information, which is also in colour (Lillesand et al., 2015). Direct digitizing was used for delineation of sand dunes. The on-line tool, namely Google Timelapse (http://earthengine.google.com/timelapse), was used to compare satellite images of the study area as it looked in 2000 and 2016 (the most "recent" information is available for 2016). Sand dune delineation (see above) permitted recognition of particular dunes for the both compared time slices. A distance between their position in 2000 and 2016 was measured, which made possible evaluation of speed of dune migration. The three-decade data on wind directions from the Siwa climate station (Table 4) were used to assess the correspondence of the prevailing winds

Table 3 Specifications of the multi-spectral satellite imagery data acquired for this study. Satellite

Sensor and Spectral Resolution (mM)

Landsat 8 (2017), only one scene (Path 180/Raw 040), signed 16 bits

OLI Band 1: 0.43 - 0.45 Blue Band 2: 0.450 - 0.51 Blue Band 3: 0.53 - 0.59 Green Band 4: 0.64 - 0.67 Red Band 5: 0.85 - 0.88 Near- IR Band 6: 1.57 - 1.65 SWIR 1 Band 7: 2.11 - 2.29 SWIR 2 Band 8: 0.50 - 0.68 PAN Band 9: 1.36 - 1.38 Cirrus TIRS Band 10: 10.6 - 11.19 TIRS 1 Band 11: 11.5 - 12.51 TIRS 2 http://earthexplorer.usgs.gov/ http://glcf.umd.edu/

Sources

Spatial resolution (m)

Band

30 30 30 30 30 30 30 15 30

Swath (km)

Scene size (km*km)

Altitude (km)

185

170  185

705

100 100

Table 4 Wind data from the Siwa climate station, 1971–2001 (extracted from El Nagar, 2008; Khedr, 2008). Direction (percentages) Month

N

NNE

ENE

E

ESE

SSE

S

SSW

WSW

W

WNW

NNW

Quiet

Different directions

Average speed (km/h)

January February March April May June July August September October November December Annual Average

3.5 3.8 3.7 5.3 5.6 9.3 10.6 10.8 7.8 5.2 3.3 2.7 5.97

3.6 4.7 6.5 8.9 12 15.5 18.9 17.5 15.9 11 6 3.4 10.3

4 5.5 6.8 9.9 13.7 12.3 11.2 10.5 13.3 12 6.3 4.6 9.18

5.7 8.1 11.7 14.7 15.3 11 6.6 6.6 8.9 9.6 7.1 6.3 9.3

6.7 8.5 11.7 12.1 11.7 7.2 3.1 3.1 4.8 7.5 7.7 7.1 7.6

5.6 6 6.9 6 6 6.4 3.7 1.4 1.3 2.9 3.9 5.8 5.6

4.7 5.6 4.2 4.1 3.1 2.1 1 1.2 2.2 3.2 4.7 5.3 3.45

5.5 5.1 3.7 3.4 2.3 1.9 1.2 1.3 2.3 3.5 5.3 5.8 3.44

10 6.8 5.5 3.1 2.5 2.8 2.5 2.7 3.2 4.3 7.5 9.1 5.03

20.1 16.4 11.6 6.6 4.9 6.2 6.4 7.4 6.1 8.1 14.4 17.6 10.5

15.5 14.4 13.9 11.6 8.5 10.1 13.4 14 11.8 12.3 13.6 15.5 12.9

8.9 10.2 10.2 10 10.8 14.6 20.6 20.7 16 11.7 9.1 7.6 12.5

5.9 4.6 2.8 3.8 2.6 2.9 2.3 2.5 4 7 8.6 8.9 4.66

0.3 0.3 0.8 0.5 0.6 0.4 0.4 0.4 0.8 0.7 0.6 0.5 0.53

10.5 11.8 13.9 14.4 12.6 11.7 11.1 9.79 8.88 8.08 7.58 9.42 10.8

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geodiversity hotspot (the latter corresponds to the entire oasis boasting by rich and concentrated geoheritage). At all localities, sand dune activity has been registered visually. The outcrops are surrounded and partly submerged by sand. Sand occupies significant part of locality A (Fig. 3A). Moreover, its migration over outcrops leads to denudation and destruction of the exposed rock. Fossil shells and iron concretions are detached from the parent rock and mixed with sand, i.e., these are reworked and shifted to Quaternary sediments (Fig. 3B). The same situation is documented at locality B where sand dunes tend to cover outcrops completely (Fig. 4A, B). Undoubtedly, some exposures of the Marmarica Formation at this locality are now covered by sand masses. Finally, the situation is similarly challenging at locality C where sand dominates the space; natural outcrops look like isolated "islands" above the surface of sand, and these "islands" are so low that they can be submerged quickly (Fig. 5A, B). The field observations described above provide clear evidence of geoheritage loss to the south of the Siwa Oasis because of sand dune migration. This loss occurs in two modes. The first is submergence of natural outcrops by sand. It is registered at all localities, but it seems to be the most severe at the locality B. The second mode is destruction of natural outcrops through removal of fossil shells and iron concretions from the rocks and their incorporation with the overlying sediments. The loss of the second mode is registered chiefly at the locality A. 4.2. Remote sensing evidence of geoheritage damage

Fig. 3. Locality A: A – general view, B – close view of fossils and iron submerged by sand.

abundance. This is demonstrated by El-Sabbagh and El Hedeny (2016), the results of whose investigations at the similar locality are internationally important. Shell beds are also important in geoscience education (visiting the Siwa Oasis is included occasionally into educational excursions for students in Earth sciences of some Egyptian universities), as well as being attractions for ordinary visitors (geotourists) interested in seeing shells (like on a modern seashore) in desert environment (the Siwa Oasis gains popularity among tourists, and, thus, attraction of geotourists to this remote destination is not a problem). Generally, the geological uniqueness of the three analyzed localities is a result of three things. First, they represent the Marmarica Formation with a high geoheritage potential. Second, these localities have a range of unique features. Several geoheritage types can be distinguished there (Table 1). The sedimentary types refers to shell beds and iron concretions, the palaeontological type refers to abundant bivalves, the palaeogeographical type refers to specific facies and trace fossils, and the geomorphological type refers to small natural arches. Third, these are, probably, the first localities of the Marmarica Formation reported from the south of the Siwa Oasis. The rank of these geoheritage sites is no more than regional in regard to the occurrence of the similar features in Egypt. Importantly, similar localities can be found elsewhere to the south of the Siwa Oasis, but these studied seem to be the first providing the evidence of the local distribution of the Marmarica Formation, and, thus, their uniqueness will remain undisputable. These localities extend the scope of the geoheritage of the entire Siwa Oasis (Sallam et al., 2018) and serve as sites illustrating the

Remote sensing techniques help to illustrate the role of sand dune migration as a factor of geoheritage loss in the Siwa Oasis. Satellite images demonstrate the existence of multiple linear sand dunes on the study area (Fig. 6). Apparently, these dunes are oriented across the prevailing northwestern direction of wind (Fig. 7). The comparison of satellite images for 2000 and 2016 implies migration of dunes to the southeast, which corresponds to the prevailing winds (Fig. 7), with a speed of ~10 m/yr. The three geoheritage localities considered in the present study are situated in the pathway of sand dune migration (Fig. 6), and these may soon be submerged with sand. The situation is especially dangerous in the case of the locality B. The latter is situated very closely to a sand dune (Fig. 4A). The distance between the fossiliferous outcrops and the dune is~10 m. Considering the position of this locality relatively to the dune and taking into account the established direction and speed of dune migration, it is clear that the locality will be submerged by sand (at least partly) within one–two years, which means direct geoheritage loss in the immediate future. Risks are lower in the cases of the localities A (Fig. 3A) and C (Fig. 5A) because the distances between them and the nearby dunes are much larger. However, sand dune migration to the south of the Siwa Oasis is strong enough to hypothesize submergence of these localities within a few decades. The results of the remote sensing provide further evidence of geoheritage loss because of sand dune migration, but over a larger area. Some features related to natural rock exposures will disappear within a short time (a few years), and some will disappear later (a few decades). On the one hand, this will lead to loss of information on unique features. Moreover, some of these features will never be recognized as such because of sand submergence before a geoheritage inventory taken place. On the other hand, sand dune migration does not only cover geoheritage sites making them invisible, but also damages them physically (see sub-section 4.1). Active destruction of rocks and removal of fossils erases rare geoheritage objects leading to irretrievable loss.

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Fig. 4. Locality B: A, B – general view and submergence by sand (arrows indicate directions of submergence), C, D – close view of geological features submerged by sand (the size of the arch is~1 m).

5. Discussion 5.1. Conceptual discussion The results of this study imply that sand dune migration is a significant factor in certain geoheritage loss in the Siwa Oasis (chiefly static elements like natural exposures of the Marmarica Formation are subject to this loss). On the one hand, this factor is responsible for denudation and destruction of geological exposures. On the other hand, it leads to submergence of these exposures by sand. It is very possible that potentially important geoheritage sites have been submerged already by sand to the south of the Siwa Oasis. More generally, the same threats apply to those large parts of the Sahara Desert where sand dunes move actively. These statements raise significant questions about geoheritage management in the face of these threats: 1) is it sensible (if possible) to resist the natural process with protection of some local geoheritage? (theoretical question) 2) which geoconservation procedures are suitable to sandinfluenced localities of the Siwa Oasis? (practical question) 3) is local protection of geoheritage from sand dune migration adequate? (question of regional planning) The first question is most challenging. On the one hand, geoheritage loss in a geodiversity hotspot is a real danger to unique geoheritage itself and to the local socio-economic growth based on this important resource. On the other hand, the rank of the geoheritage localities considered in this paper are relatively low (but they are of heritage value anyway), and similar localities can be found elsewhere in the Siwa Oasis, including its northern part

(El-Sabbagh and El Hedeny, 2016); moreover, sand dunes themselves constitute a high-value geoheritage (Sallam et al., 2018). This is a true dilemma that can be resolved taking into account two considerations. First, palaeontological, palaeogeographical, and other geoheritage localities of the Siwa Oasis are yet to be inventoried, and, thus, it is impossible to know which of them are really precious. Second, such localities (even if lower-ranked) are really few in comparison to the spatial distribution of sand dunes (Fig. 6). The combined information from all available palaeontological, palaeogeographical, and other localities makes the relevant geoheritage types of the oasis really unique. If so, protection of geoheritage localities from sand dune migration is urgent task for the Siwa Oasis. The second question requires specification of a two-folded system of actions. First, all geoheritage features of the Siwa Oasis should be identified, mapped, evaluated, and catalogued. This risk of geoheritage loss linked to sand dune migration should be evaluated for each of them. This will permit identification of the priorities for geoconservation (cf., Reynard and Brilha, 2018). If one unique phenomenon (the Marmarica Formation in this case) occurs in numerous localities, the most representative of the latter should be protected at first. However, it is necessary to understand that if nationally- or globally-unique geological phenomena (i.e., rare on the scale of Egypt and the world, respectively) concentrate in the Siwa Oasis, all localities representing each of these phenomena require protection from possible damage, either anthropogenic or natural. Second, geoconservation procedures have to be developed. "Classical" geoconservation has developed in Europe where environmental conditions differ from those of the Sahara (Prosser et al., 2006; Reynard and Brilha, 2018). Some simple actions can be implemented in arid regions – for instance,

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geoheritage loss because of dune migration. Even if this investment is not enough to secure full protection of the local geoheritage from sand submergence, geopark creation will facilitate geoheritage-related decision-making, which is necessary for the timely reaction to natural threats. However, even geopark creation may be not enough because desertification in the Siwa Oasis is a large-scale process, the underlying cause of which is linked to problems of water use in the oasis (Ghonaim and Gabriel, 1980; Döring and Nuding, 2002; Abdulaziz and Faid, 2015; Aly et al., 2016; Powell and Fensham, 2016). A more general and highly-complex program is necessary to improve the existing water use strategies and infrastructure in the entire oasis for the purposes of local sustainable development. In such a case, the very presence of significant geoheritage resource in the oasis is among arguments for such a program. The latter should take into account the needs of geoconservation. Normal functioning of a desert oasis leads to its sustainable development (Fassi, 2017), which facilitates protection of local geoheritage from natural threats, the biggest from which is sand dune migration. 5.2. Pragmatic discussion Consideration of the earlier discussions of geoheritage management under the natural threats (Sharples, 2011; Brown et al., 2012; Wignall et al., 2018) implies four possible options for the studied localities of the Siwa Oasis: 1) to do nothing (to allow submergence by sand); 2) to rescue and to record (documentation for further notions and possible re-excavation); 3) to locate replacement sites (finding alternative sites in the vicinities of the Siwa Oasis); 4) to fix and to control (protective technical solutions).

Fig. 5. Locality C: A, B – general view and "remains" of natural exposures after severe submergence by sand.

regular cleaning of the site (from sand in this case) and installation of fences or protective walls. Of course, sand dunes may be too big relative to geoheritage sites (Fig. 4A), which makes these actions impractical. In this case, more advanced technical solutions are required (such as high wall construction). The third question is highly-complex. It is evident that geoheritage loss minimization in the Siwa Oasis will be difficult to address with only local actions like cleaning and fence installation. On the one hand, geoheritage manifestations in the oasis are multiple (Sallam et al., 2018; see also text above). One the other hand, geoconservation procedures, especially protection from sand submergence may be costly. In regard to these issues, a regional geoconservation plan is required (see Prosser, in Reynard and Brilha (2018) for a detailed explanation), as well as attraction of funds for its implementation. A suitable solution is creation of a geopark with subsequent geotourism growth, as earlier recommended by Sallam et al. (2018). Geoparks are very efficient for geoconservation, geoheritage promotion for tourism purposes, and regional sustainable development (Henriques and Brilha, 2017). These establishments allow coordination of geoheritage management on a local-to-regional scale and promote geoheritage as a really profitable natural resource to policy-makers and the general public. Geoparks are also able to attract significant investment and to maintain infrastructure, which are required for resistance to

The choice depends evidently on the balance between geoheritage sites value (uniqueness) and cost of works. The rank of the studied localities is not more than regional, which means high costs for their conservation are unreasonable. However, geoheritage to the south of the Siwa Oasis is not so diverse (Sallam wt al., 2018), and the loss of even low-valued localities may really shrink the geodiversity hotspot. For now, the most important task seems to be geological re-mapping of the southern periphery of the oasis with the subsequent inventory of geoheritage sites. If other sites which represent the Marmarica Formation adequately and are not influenced by sand dune migration are available, the studied localities can be 'abandoned'. However, this option should be taken with caution in regard to the distance of geoheritage sites from the oasis. If this distance is too big, the integrity of the entire oasis geoheritage will be lost. One may argue that sand dunes themselves and the process of their migration are notable geological phenomena that determine their uniqueness; and if this uniqueness higher than that of the three analyzed localities, the 'do noting' approach seems to be preferable option. Although the Great Sand Sea to the south of the Siwa Oasis is really of significant uniqueness (Sallam et al., 2018), the above-given argument is weak because of incomparable spatial dimensions of the objects. The Great Sand Sea occupies vast territory whereas the exposures of the Marmarica Formation are very small. The loss of one such exposure means significantly bigger damage than the hypothetic loss of any single dune. Development of a geoheritage management plan/strategy for the Siwa Oasis that is considered above is vitally important. However, achieving this task may require up to several years, whereas sand dune migration and the relevant geoheritage loss occur faster (see above). This is a serious challenge making the 'rescue and record' option most pragmatic for today. However, the

Please cite this article in press as: K.M. AbdelMaksoud, et al., Sand dune migration as a factor of geoheritage loss: Evidence from the Siwa Oasis (Egypt) and implications for geoheritage management, Proc. Geol. Assoc. (2019), https://doi.org/10.1016/j.pgeola.2019.07.001

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Fig. 6. Distribution of linear sand dunes used to establish the speed of dune migration (see text for explanation and data source).

preference of this option does not mean that the solution should be postponed for significant time. Sand dune migration may affect some other geoheritage sites (including those yet to be discovered), and the plan development would allow these to be saved. It is important to add that the studied localities are small sized, and, thus, their protection from sand dunes may require relatively low-cost technical solutions. The main task is finding the latter. Sand is a significant factor of archaeological heritage loss, which has been documented in both North Africa and many other regions of the world (Camuffa, 1993; Hussein and El-Shishiny, 2009; Daniel et al., 2013; Roy and Sethumadhav, 2014; Hilton et al., 2018). Modern archaeology is familiar with techniques for excavation and protection of sites submerged by sand (e.g., see the example of Sphinx conservation – https://www.guardians.net/hawass/ sphinx2.htm), although this erosion is not as fast as submergence by dunes, and, thus, there is a time to address this problem by geoheritage management plan development. 6. Conclusions This study in the Siwa Oasis of the Sahara characterizes the mechanism of geoheritage loss because of sand dune activity and leads to three general conclusions:

Fig. 7. Wind directions (annual average) in the Siwa Oasis (see data in Table 4).

1) sand dune migration triggers physical damage of unique geological localities, as well as their submergence by sand;

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